Method for displaying gaze point data based on an eye-tracking unit

ABSTRACT

A method of presenting gaze-point data of a subject detected by an eye-tracking unit includes presenting a test scene picture acquired by a camera unit, and displaying shapes on the test scene picture. The shapes represent momentary gaze points of the subject.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 13/145,636, filed Jul. 21, 2011, now U.S. Pat. No. 9,495,589,which is a national phase application of PCT Patent Application No.PCT/EP2009/000480, filed Jan. 26, 2009.

FIELD OF THE INVENTION

The invention generally relates to human eye-tracking devices and moreparticularly relates to a gaze-point detection system which is assistedby reference signals emitted by optical sources provided in the sceneunder viewing.

BACKGROUND

Monitoring or tracking eye movements and detecting a person's gaze point(as used herein, the point in space at which the person is looking) canbe used in many different contexts. Such measurements can be animportant information source in analysing the behaviour andconsciousness of the person. The information can be used both forevaluating a test scene (as used herein, the visual percept of a regionof space in which visible objects or surfaces may be present and insidewhich a gaze point can be detected) observed by a person, and forevaluating characteristics of the respective person. The diverse uses ofgaze point detection include studies on the usability of software anddifferent types of interfaces; evaluation of web pages, advertising andadvertisements; provision of means for educating pilots in simulatorenvironments and for training surveillance personnel insecurity-critical roles; and research in psychology, behaviouralsciences and human perception. A field which has attracted an increasinginterest in recent years is the evaluation of advertising and othermarketing channels. Eye-tracking information may then be gathered from aviewing test person's examination of advertising of a particularmerchandise, and the response of the test person is derived.Eye-tracking devices may also be used for collecting information on theresponse of the viewing person of the placement of a particular articleon a shelf of a shop display.

In addition to these applications, there are interactive applicationswhich employ information about the area at which a person is looking inorder to respond or react in different ways. For example, theadvertising or display of goods in a shop window can undergo a visualchange responsive to the detected fact that a person is looking at it.Moreover, a computer system may utilise continuous information on thegaze point of a user to be better capable of displaying the object inwhich a user is interested and of adapting it intelligently to differentbehaviours of the user. Thus, an eye-tracker could form part of aninterface for human-computer interaction. Since the content of thedisplay is programmed in software, software can also correlategaze-point information with the semantics of the program.

While eye-tracking-assisted studies of human behaviour up to now havebeen concerned with a limited number of individuals, many marketresearchers wish to carry eye-tracking technology over to thestatistical, high-volume approach which has historically been verysuccessful in this field. For instance, the cumulated reactions to aparticular advertisement campaign of a large number of test subjects mayprovide an accurate prediction of the efficiency of this campaign duringits future, full-scale exposure in society. This emerging use of eyetrackers leads to new challenges on the technological level. Forinstance, available eye trackers are usually complex and fragiledevices, and their operation may be demanding—at least to a user havinga background in areas such as marketing studies or behavioural sciencesrather than a relevant technological field. The applicability ofeye-tracking technology to high-volume studies is limited by two factsin particular. Firstly, adapting an eye tracker to a new person may be atime-consuming and technologically demanding exercise, and secondly,many potential test subjects may feel hesitant or reluctant to usingsuch a device if its interface is complicated or physically intrusive.

As seen above, for an equipment to be suitable for gaze-point detectionin group studies, it is important that it can switch between the testsubjects quickly and conveniently. On a different level, theeye-tracking equipment should also be easily deployable in differentenvironments, and the visual objects to which the test subjects are tobe exposed should not be limited to, e.g., computer monitors. Indeed,from the point of view of marketing research, eye-tracking-based studiesmay be of interest in any environment where consumers receive visualinformation on which to base their buying decisions, and the range ofpossible psychological experiments to which eye-tracking technology isapplicable seems unlimited.

In conclusion, there appears to be a need for an eye-tracking systemwhich solves at least some of the problems discussed above.

SUMMARY OF THE INVENTION

An object of the present invention is to present gaze-point data of atleast one subject detected by an eye-tracking unit.

A method of presenting gaze-point data of at least one subject detectedby an eye-tracking unit may comprise presenting a test scene pictureacquired by a camera unit, and displaying shapes on the test scenepicture. The shapes may represent momentary gaze points of the at leastone subject.

The shapes may comprise circles, dots, stars, rectangles and/or crosses.A size of each shape may vary to indicate accuracy of the gaze-pointdata.

The momentary gaze points may be acquired by the eye-tracking unit overa time interval of non-zero length. The method may further comprisedisplaying textual annotations specifying a respective time of viewingof the momentary gaze points. The method may further comprise displayinglines or arrows connecting the shapes to specify an order of themomentary gaze points.

A color of the shapes may indicate a dwell time of the at least onesubject's gaze. The shapes may represent a sum of dwell times of aplurality of subjects.

The method may further comprise predefining a plurality of areas ofinterest in the test scene picture. The dwell time of one or moresubjects in each area of interest may be accumulated.

The method may further comprise superimposing shaded areas of interestin accordance with statistical measures of the dwell times of the one ormore subjects. The statistical measures may comprise a mean total dwelltime per test subject, a mean duration of each fixation, percentiles ofthe total dwell time and/or percentiles of a test population havingvisited each area of interest.

A video sequence may be constructed based on the test scene picture incombination with an IR signal source tracking data provided by the eyetracking unit by applying perspective transformations.

A plurality of test scene pictures acquired at regular intervals forminga video sequence may be presented. Shapes on respective frames of thevideo sequence may also be displayed.

Another aspect of the present invention is to provide a device fordetecting gaze point in a test scene. Such a system may comprise atleast four functionally separate parts:

one or more infrared (IR) signal sources to be placed in a region ofspace (test scene) currently studied for use as spatial referencepoints;

at least one pair of eye glasses to be worn by a person (test subject)and having the double capability of detecting the IR signals andtracking eye movements of the person;

a data processing and storage unit for calculating a gaze point of theperson; and

a scene camera for acquiring a picture of the test scene.

The scene camera may be an external device or may be integrated in theeye glasses as a camera unit. Although the inventors consider these twoembodiments equally useful, the present disclosure will focus on thecase where the scene camera is integrated in the eye glasses. This isnot intended to limit the scope of the invention, nor to indicate apreference towards embodiments having this characteristic. In fact, mostreferences to ‘camera unit’ may quite simply be replaced by ‘scenecamera’ where appropriate.

Output of a system according to the invention includes a signalindicative of a gaze point of the test subject. The gaze point ispresented relative to a picture of the test scene, in which one or moreIR signal sources have been positioned and are operable to emit IRlight. Each of the one or more pairs of eye glasses has an image sensoradapted to receive the IR signals from the at least one IR signal sourceand to generate an IR signal source tracking signal corresponding to thedetected IR signals. Each pair of eye glasses secondly includes aneye-tracking unit adapted to determine the gaze direction of an eye ofsaid person and to generate an eye-tracking signal corresponding to thedetected gaze direction. Thirdly, each pair of eye glasses comprises acamera unit (unless this is embodied as an external scene camera)adapted to acquire at least one picture of the region of space, relativeto which the gaze point is determined. The data processing and storageunit is adapted to communicate with a pair of eye glasses according tothe invention, so as to obtain the at least one IR signal sourcetracking signal (from the image sensor), the eye-tracking signal (fromthe eye-tracking unit) and the at least one picture (from the cameraunit). The data processing and storage unit is further adapted tocalculate a gaze point of the person with respect to one of said atleast one acquired picture as a function of the at least one IR signalsource tracking signal, the eye-tracking signal and the at least onepicture. The gaze point may then be indicated directly in the picture ofthe test scene, which is an intuitive and ready-to-use presentationformat.

A system according to the invention is easy to set up, is failsafe andprovides accurate gaze-point data. Part of the robustness of themeasurements is owed to the eye glasses' capability of jointlydetermining the locations of the IR signal sources and the gazedirection of a person wearing the glasses. The camera unit of the eyeglasses (or, respectively, the external scene camera) is adapted toprovide a picture of the test scene, relative to which a gaze point canbe calculated. Preferably, the eye-tracking unit and the image sensorare rigidly connected to the eye glasses or to one another. This isbecause their measurements are performed in separate reference frames,which would lack a stable interrelation if their relative orientationsand positions were allowed to change between or during measurements. Inalternative embodiments, wherein the eye-tracking unit and the imagesensor are mobile with respect to one another, there are providedposition and orientation indicators or functionally equivalent devicesto furnish the information needed for interrelating the referenceframes.

The IR signal sources, at least when arranged in a protective housingand provided with suitable fastening means, can be deployed in a verywide range of imaginable test scenes, which provides a high degree ofversatility. Moreover, most test subject are accustomed to sun glassesor other spectacles and therefore feel comfortable when wearing the eyeglasses of the inventive system. The latter do contain sophisticatedtechnological devices, but have the familiar appearance of a regularpair of eye glasses, allowing a test subject to relax and behavenaturally during measurements.

The system is scalable by virtue of the possibility of providingmultiple pairs of eye glasses, for a limited number of available sets ofindividual equipment may be a bottleneck in studies comprising a largenumber of test subjects. It is expedient in such studies to providewireless communication links between eye glasses and the rest of thesystem. The system also has a favourable scalability on the dataprocessing level, for the data processing and storage unit can easily beadapted to calculate the gaze points of all test subjects relative toone picture of the test scene. This is an intuitive and immediatepresentation format and also avoids the tedious task of manuallycomparing different view of the test scenes in order to summarise gazepoint information for different test subjects. In this connection, itshould be emphasised that the three functional parts of the system arenot necessarily physically separate. Following the general tendencytowards miniaturisation and weight reduction, the data processing andstorage unit may be physically embodied in an IR signal source, in apair of eye glasses or in some other component of the system. It is alsopossible to combine the image sensor with a camera unit or externalscene camera that is sensitive for IR radiation; most silicon-baseddigital cameras have good responsivity in the near IR spectrum. Inembodiments adapted for use in studies comprising a large number of testsubjects, however, the data processing and storage unit is preferablycentralised for the sake of scalability. This way, although the datahave been gathered in a distributed fashion (possibly using severalpairs of eye glasses), the data are ultimately accumulated in onelocation for further processing. In particular, although the cameraunit(s) are capable of acquiring a unique picture of the test scene foreach test subject, the calculated gaze points of different test subjectsare most useful when presented relative to one picture of the testscene. In embodiments where the data processing and storage unitsconsists of physically distributed sub-units, these latter make acollective decision on which picture to use.

There are further provided, in accordance with a second and third aspectof the invention, a method for detecting a gaze point of a personrelative to a picture of a region of space and a computer programproduct.

In one embodiment, the data processing and storage unit is adapted toproduce a combined picture of the test scene and the positions of the IRsignal source(s). The positions of the IR signal sources are useful asprecise reference points in the coordinate system of the picture, sothat the detected gaze point(s) can be represented with high accuracy.

The system may be adapted to utilise other reference points in additionto the IR signal sources in determining the position of the eye glassesrelative to the test scene. In particular, tracking of image features,such as edges, corners and ridges, may meritoriously complement trackingof the IR signal sources. This is especially useful after the initialdetection of the positions of the IR signal sources, so that thepositions of such image features can be determined relative to the IRsignal sources.

The IR signal sources may emit modulated IR light for thereby beingdistinguishable from one another. In other words, the IR signal sourcestransmit multiplexed signals over the IR spectrum. This facilitatesprocessing of the IR signal source tracking signals generated by theimage sensor. Suitable modulation schemes include modulation withrespect to time, such as frequency modulation, pulse-width modulationand modulation by orthogonal codes. Alternatively, the IR signal sourcesare synchronised and adapted to emit signals in separate time slots of arepetitive time frame. As another alternative, optical modulation can beused, wherein the IR signal sources emit at different wavelengths, suchas 750 nm, 850 nm and 950 nm, and absorption filters or dielectricfilters are provided at the IR sensors to separate the sources. As yetanother alternative, IR signal sources can be made distinguishable byhaving different polarisation characteristics.

The IR image sensor of the eye glasses may comprise a sensor surface,which is preferably plane, and which is arranged at some known distancefrom an optical aperture, such as a fixed or variable diaphragm, of theimage sensor. This way, all light rays incident on the sensor surfacewill pass essentially through a common point, namely the centre of theoptical aperture in the case of a pinhole camera model. In Gaussianoptics, nodal points define the incident angles of the IR beams. It isfurther possible to detect a position of the signal on the sensorsurface. The position of the signal may be a peak-intensity point, inwhich the maximal intensity is received or the centroid for sub-pixelresolution. Alternatively, if the sensor surface makes detections in abinary fashion with respect to some intensity threshold or if the pixelstend to become saturated frequently, the geometrical centre of theilluminated spot can be used as a signal position. On the basis of thecentre of the optical aperture, the signal position and the separationbetween the optical aperture and the sensor surface, the angle ofincidence of the light ray can be calculated. Preferably, this signal isincorporated in the IR signal source tracking signal generated by theimage sensor.

The eye-tracking unit of the eye glasses may comprise the followingsections: one or more devices adapted to emit IR or near-IR light,preferably arranged on a sidepiece of the eye glasses and directedtowards an eye of the person wearing the glasses; a detector foracquiring an image of the eye, the detector preferably being arranged ona sidepiece of the eye glasses; and an evaluation unit for processingthe acquired image of the eye and thereby determining a gaze directionof the eye. Preferably, the device for emitting IR or near-IR light toilluminate an eye of the person is directed towards the eye viareflection on one of the transparent plates of the eye glasses, whichthen acts as a partial IR mirror or may have an IR-reflective coating.The detector is adapted to receive only light at the wavelength of theIR-light-emitting device, so that the eye is imaged using only activeillumination. To achieve such wavelength discrimination, a filter forremoving light in the visible spectrum, such as a (low-pass) absorptionfilter, may be provided in front of the detector. Since the describedeye-tracking unit is located closely to the eye, it would be delicate toarrange illumination and detection means coaxially, for thereby enablingeye tracking in the bright-pupil state (the familiar ‘red-eye effect’)caused by retinal reflection. Therefore, preferably, theIR-light-emitting device and the detector are arranged on separateoptical axes, so that the eye is imaged in its dark-pupil condition.

According to an embodiment of the present invention, the eye glasses areprovided with a first polarisation filter before at least one eye of thetest subject. This may be advantageous in cases where the gaze detectionsystem is used outdoors. Suitably, the polarisation filter is providedas a layer on the plates (lenses) of the eye glasses. Optionally, asecond polarisation filter, the transmissive direction of which isperpendicular to that of the first polarisation filter, is arranged infront of the detector of the eye-tracking unit. This way, the detectoris blocked from any light which has been polarised by the filter on theplates, thus amplifying the relative intensity of light from theIR-light-emitting device; external light could otherwise causeexcitation of the detector and disturb the measurements. As anadditional advantage, the first polarisation filter may attenuateuncomfortable sunlight reflexes on horizontal surfaces from reaching thetest subject's eyes. The first filter will be particularly efficient inthis respect if its transmissive direction is essentially verticalduring wear, as the vertically polarised component of such reflectedsunlight is usually faint.

To save weight and volume, certain components needed for the operationof the eye glasses can be located in a physically separate support unit,which can be hung from a belt worn by the test subject or placed in apocket. Such support unit, which is communicatively coupled to a pair ofeye glasses, may contain a voltage source (such as a solar cell orbattery), a data buffer for storing information collected duringmeasurements, a processing means for performing pre-processing of data(such as data compression or conversion to a suitable format), awireless transmitter/receiver adapted to communicate with the associatedpair of eye glasses and/or with the data processing and storage unit.

The image sensor of the eye glasses may consist of two orthogonal linesensors, the scanning directions of which are preferably arrangedparallel with the frontal plane of the test subject during wear of theeye glasses. The scanning direction of one of the line sensors may bevertical, but is preferably rotated by an angle of 3-30 degrees. Inpractical situations, it is often convenient—and sometimes necessary—toarrange IR sources aligned along a vertical or horizontal line; see,e.g., FIG. 2, wherein IR signal sources 800 c and 800 d are locatedapproximately on equal height. Two sources may then give rise to adegenerated signal if the scanning direction of the line sensor isperpendicular to such line. The disclosed arrangement of the line sensorscanning directions makes this event less probable.

According to an embodiment of the present invention, at least one IRsignal source of the system is operable in an energy-saving mode orstandby mode. Since it is important that the IR signal sources areflexible and easy to deploy in various test environments, they arepreferably battery-powered. To increase time between battery chargingsor replacements, an IR signal source may be adapted to enter theenergy-saving mode after the typical duration of a test session. The IRsignal source can be reactivated by receiving, suitably via a wirelesscommunication means, a predefined signal from a pair of eye glasses orfrom the data processing and storage unit. To further reduce energyconsumption, the IR signal source preferably comprises a passivereceiving means adapted to convert incident electromagnetic radiationinto an amount of electric energy sufficient to actuate the IR signalsource into its operation mode. The eye glasses may then comprise meansfor emitting such electromagnetic radiation.

In situations where a large number of test subjects participate in aneye-tracking-assisted study, efficient collection and presentation ofthe data are essential. Hence, a system according to the invention—andadapted with special regard to such studies—may comprise a plurality ofindividual user equipments (several pairs of eye glasses), so that morethan one user at a time can look at the test scene while being monitoredwith respect to gaze point. Most expediently, however, such amulti-subject system comprises one single data processing and storageunit, so that gaze-point information for all users is gathered in onelocation. This facilitates statistical analysis of the entire amount ofgaze-point information. Additionally, gaze-point data for all testsubjects can be conveniently presented with respect to a single pictureof the test scene, the picture being acquired by any one of the eyeglasses of the system or by an external imaging device, as appropriate.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter. Allterms used herein are to be interpreted according to their ordinarymeaning in the technical field, unless explicitly defined otherwiseherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described with reference tothe accompanying drawings, on which:

FIG. 1 is a schematic drawing of a system for detection of a gaze pointof at least one person, according to an embodiment of the invention;

FIG. 2 shows the system of FIG. 1 applied to an exemplary measurementsituation;

FIG. 3 is a schematic drawing of a system according to the invention andadapted to detect gaze points of a plurality of test subjects;

FIG. 4 is a perspective view of a pair of eye glasses for use in adetection system according to the invention;

FIG. 5 is a schematic drawing of a pair of eye glasses with anassociated support unit, according to an embodiment of the invention;

FIG. 6 is a detail view of the eye glasses of FIG. 4, wherein thedirections of the scanning directions of the line sensors are indicated;

FIG. 7 is a cross-sectional view, in the plane of an incident light ray,of an angle-discriminating image sensor according to the invention;

FIG. 8 shows an IR signal source for use in a system for detecting agaze point of at least one person, in accordance with an embodiment ofthe present invention;

FIG. 9a is a combined picture of a region of space (test scene) whichincludes the positions of the IR signal sources;

FIG. 9b is the combined picture of FIG. 9a , further includingindications of image features suitable for being tracked;

FIG. 9c is the combined picture of FIG. 9a , further includingindications of gaze points of a test subject, as measured by a systemaccording to the present invention;

FIG. 10a is a combined picture of a region of space includingindications of gaze points of a plurality of test subjects;

FIG. 10b is the combined picture of FIG. 10a , including indications ofa statistical quantity as a function of position;

FIG. 11 is a flow chart showing how information is collected, processedand transmitted between different physical or functional components, inaccordance with an embodiment of the inventive method for detecting agaze point of at least one person with the assistance of opticalreference signals;

FIG. 12 is a flow chart of an alternative embodiment of the processshown in FIG. 11;

FIG. 13 is a simplified (in so far as all items are coplanar) drawingwhich shows two sets of angles corresponding to the lines of sight tothree IR signal sources, as measured from two different positions of apair of eye glasses;

FIG. 14 shows a variant of the system of FIG. 1, wherein an externalscene camera is used for taking the test scene picture, when applied toan exemplary measurement situation.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

With reference generally to FIGS. 1-8, the constituent parts of agaze-point detection system 100 according to the invention will now bedescribed. As shown in FIG. 1, the system 100 includes a pair of eyeglasses 400, two IR signal sources 800 (although the system could haveincluded any number of IR signal sources) and a data processing andstorage unit 110. The IR signal sources 800 are adapted to emit IR lightwhich can be received by the eye glasses 400. The eye glasses 400 ontheir part are operable to communicate (uni- or bidirectionally) withthe data processing and storage unit 110.

FIG. 2 shows the same system 100 applied to an exemplary measurementsituation. The eye glasses 400 of FIG. 1 are worn by a person (testsubject) 210. The eye glasses 400 are adapted to fit snugly onto theperson's 210 head 215, so that little relative movement is possible. Ifthe eye glasses 400 are overly mobile with respect to the head 215 ofthe person 210, then the accuracy of the gaze-point measurements can bethreatened, especially in case of sudden head movements or vibrations.Slow movements, however, usually do not cause any problems, since theeye-tracking unit repeatedly measures the position of the eye along withthe gaze direction. As indicated in FIG. 2, the IR light generated byeach of IR signal sources 800 is not concentrated in a narrow ray but isemitted over a wide solid angle, such as a half-sphere. This allows thetest subject 210 to be positioned in a variety of points on the ground,and also avoid the need for readjustment of the IR signal sources 800if, for instance, a tall test subject follows a short one, or if sometest subjects prefer to watch the test scene from a sitting positionwhile others rather stand. The data processing and storage unit 110 canbe embodied as a general-purpose computer with computing and storagecapabilities as well as communication means adapted to connect to otherconstituent parts of the system, particularly eye glasses, in order toreceive and/or transmit data.

In the exemplary situation shown in FIG. 2, four IR signal sources 800are arranged in a bookcase 220 containing visible objects: a sphere, anoctahedron, a clock, plants, a vase, various books and rectangularboxes. The bookcase with its objects is the test scene of this exemplarysituation. The system's 100 presentation of the gaze point of the person210 is related to the plane in which the IR signal sources 800 arelocated, which approximately corresponds to the front plane of thebookcase 220.

FIG. 3 shows a gaze-point detection system 300 adapted to be used by alarge number of test subjects. Like the system 100 of FIG. 1, itincludes a data processing and storage unit 110 and a plurality of IRsignal sources 800. To expedite the data collection by allowing severaltest subjects to be measured at one time, the system 300 comprisesseveral pairs of eye glasses 400, which are functionally equivalent butmay differ as regards size, colour etc. As already noted, the IR signalsources 800 emit IR light in a multidirectional fashion, so that thetest subjects on which measurements are currently being performed arefree to choose their viewing positions within a reasonably large areafacing the test scene.

FIG. 4 depicts details of the eye glasses 400, the main tasks of whichare to sense the gaze direction of a person wearing them, to receive IRreference signals from the direction of the test scene and to acquire apicture of the test scene. In order to enhance data quality, it isimportant that the eye glasses 400 are not too mobile with respect tothe head of the person during a measurement session. When the eyeglasses 400 are worn, sidepieces 404, 406 of the eye glasses 400 rest onthe ears of the person, and nose plates 410, 412 rest on an upperportion of the person's nose. Preferably, the sidepieces 404, 406 aresomewhat elastic and exert a small inward force, thereby keeping the eyeglasses 400 in place during movements of the person's head. The designof the sidepieces 404, 406 is of little consequence to the operationsassociated with the gaze-point detection, the one shown in FIG. 4 beingintended as an example. Preferably, the nose plates 410, 412 are of suchshape and material that the lateral mobility is limited and annoyingslipping of the eye glasses 400 down the nose is prevented. The eyeglasses 400 also comprise visually transparent plates 402, 408. One actsas a mirror for an IR detector (not shown) adapted to image an eye ofthe person, whereas the other could in principle be omitted, for unlessthe intended wearer of the eye glasses 400 suffers from some defect ofvision, the visually transparent plates 402, 408 have zero refractivepower. An IR mirror is deposited on at least one of the visuallytransparent plates 402, 408 using a stack of thin film coatings havingnegligible interference in the visible spectrum. The mirror is depositedon the side facing the eye during wear. The plates 402, 408 add someweight to the eye glasses 400, increasing their mechanical stability.They may also serve a pedagogical purpose in so far as the wearerinstinctively positions the eye glasses 400 according to his or herhabit of wearing other eye glasses. Particularly, a correct position ofthe eye glasses 400 implies that the eye-to-plate distance is notuncomfortably small and that the plates 402, 408 are orthogonal to arelaxed gaze direction of the person, thus essentially parallel to thefrontal plane FP (cf. FIG. 6) of the wearer. A projection 418 on theright side piece 406 may contain equipment (not shown) associated withthe data gathering, such as an evaluation unit, pre-processing means, avoltage source and means for communicating with other components of thesystem 100.

The eye glasses 400 comprise several measuring devices. Firstly, aneye-tracking unit (not shown) is provided. As a part of the eye-trackingunit (not shown), an IR light source 420 for illuminating the right eyeby invisible IR light is provided on the projection 418. The eye glasses400 shown in FIG. 4 are adapted to track the gaze direction of the righteye, but a variant for tracking the left eye, or both, would be equallyuseful. A detector (not shown), which is too provided on the projection418, is adapted to repeatedly acquire (via reflection off the rightplate 408) an image of the right eye illuminated by the light source420. From each image, information can be extracted for determining thegaze direction. This information may be location of the light-sourcereflection (glint) on the cornea and the position of the pupil or, afterprocessing, the location of the cornea and the orientation of the visualaxis of the eye. Extraction of the gaze point direction, which may beperformed by an evaluation unit (not shown) if such is provided, may beeffectuated according to any of the algorithms known in the art. Thegaze direction can be expressed in terms of linear coordinates in atwo-dimensional image plane which moves with the eye glasses; then, moreprecisely, the gaze direction is the point at which the visual axis ofthe eye intersects this image plane. To facilitate subsequent dataprocessing, the locations of the IR signal sources are suitablyexpressed in the coordinate system of the same image plane.Alternatively, the detector (not shown) could have been arranged on theprojection 418 or on a portion of the sidepiece 406, from which the eyeis visible. Care should be taken, especially if the detector (not shown)is arranged on the projection 418, that the detector is not coaxial ornearly coaxial with the light source 420. This could otherwise cause aretinal reflection, leading to an undesired bright-pupil effect in somegaze directions. The measurement data from the eye-tracking unit (notshown) are output as an eye-tracking signal.

Secondly, an image sensor 416 is provided on a frontal side of the eyeglasses 400, suitably under a protective transparent plate. Thetransparent plate may be adapted to block shorter wavelengths, and maybe a low-pass filter, alternatively a band-pass filter centred aroundthe IR signal centre wavelength. The image sensor 416 is adapted toreceive at least one IR signal for use as a reference in determining thelocation of the test scene relative to the eye glasses 400. The imagesensor 416 is preferably adapted to ignore other IR signals than thoseemitted by the IR signal sources 800 of the system 100 for gaze-pointdetection; signals proper to the system can be distinguished from otherIR radiation by being modulated with respect to time, frequency orpolarisation. The image sensor 416 may not only detect the presence of agiven IR signal, but can also measure its angle of incidence on theimage sensor 416, so that the line of sight to the signal source can bederived. Moreover, modulation of the received IR light can be encoded,so that identities of IR signal sources can be determined on a laterprocessing stage, such as in the data processing and storage unit. Themeasurement data from the image sensor 416 is output as an IR signalsource tracking signal. Particulars of the image sensor 416, andsuitable variants of it, will be described below with reference to FIGS.6 and 7.

Thirdly and finally, a camera unit 414 is provided on a frontal side ofthe eye glasses, preferably under a protective transparent plate. Boththe image sensor 416 and the camera unit 414, which are immobile withrespect to the eye glasses 400, are situated closely to the (visual axisof the) eye of which the gaze direction is detected, so as to minimiseparallax errors. It is noted that the positions of the image sensor 416and the camera unit 414 may be reversed without any functionalinconvenience. The camera unit 414 is adapted to acquire at least onepicture of the test scene in the visible spectrum, which does notinclude IR radiation. Because the separation of the eye and the cameraunit 414 is small, any picture of the test scene acquired by the cameraunit 414 corresponds approximately to the visual perception of theperson wearing the eye glasses 400 at that point in time, at least ifthe person is not diverting his or her gaze from the relaxed direction.

FIG. 14 shows an alternative embodiment of the system, wherein the eyeglasses do not comprise an active camera unit. The picture (snapshot) ofthe test scene is instead acquired by an external scene camera 1400. Ina picture thus acquired, the locations of the IR signal sources areentered by a user before the final data processing step. Alternatively,the scene camera 1400 is adapted to receive both visible light and IRlight, yielding a combined image, in which both the visual percept ofthe scene and the locations of the IR signal sources can be seen.

The data collected by the eye-tracking unit (not shown), the imagesensor 416 and the camera unit 414—namely, the eye-tracking signal, theIR signal source tracking signal and the at least one picture may bestored in the eye-glasses temporarily, or may be transmitted over awired or wireless communication link to the data processing and storageunit 110 of the system 100. The measurement data are generallytime-stamped, so as to facilitate further processing and presentation.The time stamps may be generated on the basis of local clocks providedin data-generating parts of the system 100. The local clocks may besynchronised with respect to a master clock (which may be a designatedlocal clock) by transmitting a synchronisation signal over thecommunication links already present in the system 100.

In a particular embodiment, the frontal side of the eye glasses 400 isprovided with an activation signal transmitter 422, which is operable totransmit a wireless activation signal—e.g., IR, optical, acoustic andeven radio-frequency—towards an expected location of an IR signalsource. It is advantageous to use a directive activation signal such asan optical signal, which makes it possible to activate the IR signalsources in one test scene separately from those of other test sceneseven if these are arranged in spatial proximity of each other. Indeed,if this IR signal source is in a standby mode which can be interruptedby such activation signal (this is detailed below with reference to FIG.8), then the IR signal source goes back to its operational mode, whichincludes emitting an IR signal that can be used as a reference signal.

FIG. 5 shows an alternative embodiment of the eye glasses 400, whereinthe eye glasses 400 are adapted to communicate with a support unit 500,in which pre-processing and (partial) evaluation of measurement data areperformed. In this embodiment, the communication is wireless and takesplace by means of a transceiver 510 at the support unit 500 and anothertransceiver (not shown) at the eye glasses 400, which is preferablyarranged in the projection 418. Alternatively, a wired connection isprovided between the eye glasses 400 and the support unit 500. The eyeglasses 400 generally have the same measuring means as in FIG. 4,notably IR light source 420, detector (not shown), image sensor 416 andcamera unit 414. However, some components have been located to thesupport unit 500 in order to extend battery time and enhance comfort ofthe wearer. Relocating the heavier components away from the eye glassesparticularly reduces the risk of a laterally skew mass distribution thatwould otherwise call for counterbalancing which could make the eyeglasses 400 bulky and heavy. The support unit 500 may be hung from abelt worn by the wearer of the eye glasses 400. In this embodiment, thesupport unit 500 comprises a data buffer 520, an evaluation unit 530 forextracting gaze-direction information from the eye-tracking signal, anda pre-processing means 540 for converting the data to a format suitablefor being transmitted to the data processing and storage unit 110. Thesupport unit 500 further comprises a rechargeable battery (not shown), amain power switch (not shown), an actuator (not shown) for theactivation signal transmitter 422 of the eye glasses 400, and atransceiver 550 for communicating with the data processing and storageunit 110. Since the distances from the support unit 500 to the eyeglasses 400 may differ considerably from the distance to the dataprocessing and storage unit 110, the two communication links may usedifferent technologies, such as Bluetooth® technology for theshorter-range communication link and wireless local area network (WLAN)technology for the longer-range communication link. Alternativeembodiments of the support unit 500 may comprise a local storage unit,such as a memory card, for buffering data; then, particularly if thedata is ultimately transmitted to the data processing and storage unit110 in a wired fashion, the transceiver 550 may not be necessary.

A more detailed description of a currently preferred embodiment of theIR image sensor 416 of the eye glasses 400 will now be given. The imagesensor 416 is composed of two orthogonal line sensors, from the outputsignals of which the reception direction (expressible as two angles,because the apparent position of an IR signal source has two degrees offreedom) and identity of an IR signal can be determined. The receptiondirection can be expressed as two angles, because the apparent position(the projection onto the plane of the test scene) of an IR signal sourcehas two degrees of freedom. As used herein, a line sensor is adirectional light sensor, which outputs a received signal as a functionof a coordinate measured along a scanning direction of the sensor. Thus,contributions from any two light sources located at the samecoordinate—that is, on a line perpendicular to the scanning directionwill be summed by the line sensor. On the basis of signals from two linesensors that are orthogonal (or at least have distinct scanningdirections), the two angles (or, equivalently, image plane coordinates)characteristic of the reception direction can be determined. In thisembodiment, the line sensors are positioned so that their respectivescanning directions are orthogonal and are parallel with the frontalplane FP of the person wearing the eye glasses 400, as shown in FIG. 6.As an inventive feature, the scanning directions of the line sensors donot exactly coincide with the vertical A and horizontal C directions.Instead, for reasons outlined in previous sections of the presentdisclosure, it has been found preferable to arrange the line sensors ina slightly rotated fashion, so that each of the scanning directions B, Ddiffers by an angle of 3-30 degrees from the vertical A and horizontal Cdirections.

FIG. 7 is a cross-sectional view of the image sensor 416 as seen in theplane of an incident IR light ray 712 and a normal axis N of an IR-lightsensitive surface 710 of the line sensor. The scanning direction of theline sensor extends in the left-right direction of the drawing, implyingthat the sensor sums contributions from sources situated on any lineorthogonal to the plane of the drawing. Portions 702, 704 of a diaphragmdefine an optical aperture 706 into the image sensor 416. In the figure,the normal axis N of the surface 710 has been drawn through a centre 708of the optical aperture 706. For simplicity of the drawing, the sensor416 is represented according to a pinhole camera model, which means inparticular that the nodal points and the centre 708 of the aperture 706coincide. More elaborate models having separate nodal points can beconceived and implemented as a simple variation of the disclosedembodiment. The line sensor outputs a received intensity INT for eachcoordinate of the sensor surface 710, as indicated by the curve drawnunderneath. It is clear from the curve that the received intensity ismaximal at a peak-intensity point 714. The distance between the opticalaperture 706 and the light-sensitive surface 710 is known a priori. Bydetermining also the distance from the peak-intensity point 714 to theprojection 708 of the centre 708 onto the light-sensitive surface 710,the system can obtain the angle α of incidence of the incident light ray712. For sub-pixel resolution centroid calculation can be used todetermine 714. As explained in connection with FIG. 6, a second linesensor, orthogonal (within approximation) to the first line sensor 710,can be similarly provided around the same optical aperture centre 708 toprovide a second angle of incidence, thus completely characterising thedirection of incidence of the ray 712. As is known to those skilled inthe art, the direction of incidence (a pair of angles) is equivalent toa pair of coordinates in an image plane of the two sensors.

As noted in earlier sections, the peak-intensity point is but one way todefine the position of the signal on the sensor surface 710. It is alsopossible, and perhaps more suitable if two or more pixels share themaximal intensity, to retrieve the geometric centre of the illuminatedspot. The centre may be computed as a centroid weighted by the localintensity value at each pixel in the illuminated spot. The centroidposition is given by the following formula:

${x_{centroid} = \frac{\sum\limits_{i}\;{x_{i} \times {INT}_{i}}}{\sum\limits_{i}\;{INT}_{i}}},$

where x_(i) is the coordinate and INT_(i) is the intensity reading ofthe i^(th) pixel. Furthermore, if the point-spread function of thesensor is known, the received signal pattern may be de-convolved withrespect to this function in order to retrieve the pre-image of thesensor. To enhance accuracy, the darkness current of the sensor, asdetermined in a calibration step, may be subtracted prior to othersignal processing.

FIG. 8 is a diagrammatic drawing of an IR signal source 800 inaccordance with an embodiment of the invention. The source 800 has an IRlight source 810 adapted to emit time-modulated IR light and a receiver860 adapted to receive a wireless activation signal from an activationsignal transmitter arranged in the eye glasses of the system.Optionally, further modulation can be achieved by arranging a chromaticor polarising filter at the aperture of the light source 810. Amodulator 820 provides the driving voltage to the light source 810, andis itself supplied by a voltage source 840. It is beneficial to theadaptability of the system to use a rechargeable battery as a voltagesource. A relay 830 is serially connected to the voltage source 840 andthe modulator 820 and is thereby operable to activate and deactivate themodulator 820. The relay 830 can be actuated by a signal from aswitching circuit 870. The value of the switching circuit's 870 signalfor controlling the switching circuit 840 is based on data provided bythe receiver 860 and by a timer 850. For instance, the switching circuit870 may be configured to cause the relay 830 to close the circuitsupplying the light source 810 for a predetermined time interval (suchas 5 minutes, which may be the expected duration of a measurementsession) following each receipt of an activation signal. After the endof this time interval, the IR signal source 800 enters a power-savingmode. It is advantageous to use a completely passive receiver 860, sothat the IR signal source 800 does not dissipate any energy during itslatency before an activation signal is received.

With reference to FIGS. 9-12, the function of the gaze-point detectionsystem will now be discussed. Firstly, FIG. 11 is a flowchartillustrating how the collected data are processed during operation of anembodiment of the invention. An eye-tracking unit ET has a detectorwhich outputs an eye-tracking signal S1. An IR sensor IS receives IRsignals from IR signal sources of the system and outputs an IR signalsource tracking signal S4, which encodes the locations of the IR signalsources as seen from the IR sensor. Because the IR signal sourcetracking signal will be used for relating the eye-tracking signal S1 toa particular coordinate system, these signals are based on measurementsthat are simultaneous or approximately so. In the depicted exemplaryembodiment, information concerning the location of the pupil and of thecorneal reflection(s) (glint(s)) of the IR light source of theeye-tracking unit can be derived from the eye-tracking signal S1. In afirst processing step P1, the eye-tracking signal is combined withpersonal calibration data S2 and a gaze point S3 is obtained. As used inthis context, personal calibration data S2 may include:

horizontal and vertical angles between visual and optical axes of theeye,

radius of corneal curvature, and

distance between the centre of the pupil and the centre of cornealcurvature.

Computational methods for determining the gaze direction (and thus thegaze point in an image plane) on the basis of an eye image containing atleast one corneal glint are known in the art, e.g., through theteachings of E. D. Guestrin and M. Eizenmann in IEEE Transactions onBiomedical Engineering, Vol. 53, No. 6, pp. 1124-1133 (June 2006), whichis included herein by reference.

The steps having been described in the preceding paragraph (which aresymbolically represented inside the dashed rectangle in FIG. 11) are tobe performed once for each test subject. What will now be discussed isdirected to the final processing and presentation of the data, whichmay, in this embodiment, be performed on the data stream continuouslyor, in case the data are stored temporarily, at an arbitrary later pointin time. Storing the eye-tracking signal S1 temporarily and performingthe processing into gaze-point data S3 in connection with the finalprocessing steps appear to be an equivalent choice. Still, the datavolume of the eye-tracking signal S1, which may be represented as avideo sequence of IR images of the eye, is much larger than that of thegaze-point data S3, so the use of the first processing step P1 as apre-processing step may in fact be influential to the performance of thesystem. As an alternative to performing the first processing step P1prior to temporary storage, the images encoded in the eye-trackingsignal S1 may be cropped, filtered and/or compressed to save spaceand/or bandwidth.

With reference again to FIG. 11, the IR sensor and the camera unit CU(which is of course functionally equivalent to, and will not bedistinguished in this disclosure from, an external scene camera)respectively acquire an IR signal source tracking signal S5 and apicture S6 (snapshot) of the test scene at an arbitrary point in time.In a second processing step P2—which uses hardware calibration data S8relating the reference frames of the camera unit CU and the IR sensorIS—the picture S6 of the test scene and the corresponding IR signalsource tracking signal S5 are compared in order that a relationship S7between the IR signal source locations and the picture elements of thetest scene picture is obtained. To illustrate, FIG. 9a shows a pictureof an exemplary test scene (the bookcase 220 of FIG. 2) in which thelocations 910 of the IR signal sources have been visualised as doublecircles. Thus, the four locations 910 correspond to four definitepicture elements of the test scene picture S6; identifiers (coordinates)of these picture elements may be used to encode the relationship S7. Athird processing step P3 has as its input variables the simultaneouslyacquired gaze point data S3 and the IR signal source tracking signal S4.Further, parameters necessary for the computations are derived from thehardware calibration data S8 and the relationship S7 between the IRsignal source locations and the test scene picture. This thirdprocessing step P3 consists in a transformation of the gaze point S3from a moving reference frame of the eye-tracking unit into a referenceframe of the test scene picture S6. In this embodiment, the referenceframe in the test scene picture is simply the picture elements. Hence,the gaze-point is expressed as gaze point picture elements S9. In afourth processing step P4, the gaze point picture elements S9 are drawn(superimposed) on the picture S6 of the test scene, thereby yieldingdata S10 representing a combined picture, which are provided to anoutput means OUT, such as a monitor, a printer, an output file or anelectronic message. An exemplary combined picture is shown in FIG. 9c ,the gaze point being indicated by a dashed circle 930.

The transformation of the gaze point, which is performed in the thirdprocessing step P3, more precisely is a perspective projection. As thoseskilled in the art will appreciate, a perspective projection of a pointcan be expressed as a linear mapping if the point is given inhomogeneous coordinates. The perspective projection is the identitymapping if the eye glasses have not been dislocated. If instead the eyeglasses have been moved (translated and/or rotated), then theperspective projection can be defined as the mapping which transformsthe visual percept (image-plane points) of the IR signal sources at thatpoint in time when the eye-tracking data S1 were collected into thevisual percept of the IR signal sources when the test scene picture S6was acquired. FIG. 13 shows an example of how angles-of-sight to threeIR signal sources 800 in the test scene TS change, from a first triplet(α₁, α₂, α₃) into a second triplet (β₁, β₂, β₃), when the eye glassesare displaced from a first position R1 to a second position R2. FIG. 13is simplified in so far as the eye glasses are oriented identically inboth positions and all objects in the figure are coplanar; in reality,two angles to each IR signal source are measured. In this embodiment,movements of the eye glasses are not traced, so that a priori, theperspective projection is only implicitly known. Preferably, the mappingis estimated on the basis of the IR signal source tracking signal S4 andthe relationship S7 between the IR signal source locations and the testscene picture (the picture elements corresponding to the IR signalsources). The estimation may be effectuated by means of a direct lineartransformation. As the skilled person will appreciate, a completeestimation of the perspective projection requires knowledge of at leastfour distinct IR signal source locations. The estimation will providemultiple solutions if fewer IR signal source locations are known. If thetest scene picture S6 contains more than four IR signal sources, it maybe advantageous to not use the excess ones. In particular, unwanted IRsignals may appear in the test scene view as reflections or may emanatefrom sources behind the test scene. The selection of what IR signalsources to use may either be performed by a user or take placeautomatically in the system; e.g., the four most intensive IR signalsmay be selected as a part of the second processing step P2.

The precise distribution of the processing steps between differentconstituent parts of the system has not been discussed above. Storingand processing data are activities that gradually require less energyand occupy less physical space as the technological developmentprogresses. The tasks can in fact be allocated in many, fairlyequivalent ways. As an example, the allocation shown in table 1 may beapplied.

TABLE 1 Allocation of processing tasks Unit Processing step(s) Eyeglasses — Support unit P1, P2 IR signal source — Data processing andstorage P3, P4 unit

The distribution shown in table 1 may not be optimal if the followingassumptions are true: (i) communication links between the eye glassesand their respective support units are slow and unreliable, as arecommunication links between the support units and the central dataprocessing and storage unit; and (ii) numerical computations can beexecuted at the eye glasses at low battery consumption. In suchconditions, the alternative allocation scheme set forth in table 2 mayperform better.

TABLE 2 Alternative allocation of processing tasks Unit Processingstep(s) Eye glasses P1, P2 Support unit P3 IR signal source — Dataprocessing and storage P4 unit

By routine experimentation, the skilled person will find a suitabledistribution of the processing steps in a given situation once thisspecification has been read and understood.

FIG. 12 illustrates an alternative embodiment of the process shown inFIG. 11. Advantageously, this embodiment is not dependent on the IRsensor IS receiving signals from all IR signal sources at every instantthat an eye-tracking signal is generated. To compensate for one or morelost IR signals, it is required that the camera unit CU has acquired apicture of the test scene when the eye-tracking signal was generated. Asa simplified example, it is assumed that an eye-tracking signal S1 and atest scene picture S11—but no IR signal—were received at one point intime. Prior to this point in time, a complete set of IR signal sourcesS5 have been received and located in the test scene picture S6, like inthe second processing step P2 in the embodiment described above.Additionally in this alternative embodiment, image features areextracted from the picture and located and stored for later use. Thelocating of IR signal sources and of extracted image features, thetotality of which is denoted by S13, is effectuated in an alternativesecond processing step P5. The output S13 can be visualised as in FIG.9b , wherein the previously shown IR signal source locations 910 arecomplemented by image feature locations 920. In FIG. 9b , the followingimage features have been extracted: a top corner 920 a of theoctahedron, a support 920 b of the clock, a top left corner 920 c of theleftmost book, and an intersection point 920 d of various structuralelements of the bookcase 220. It is assumed that these four imagefeatures have been extracted and located in both the test scene pictureS6 and in a more recent test scene picture S11, namely one having beenacquired simultaneously with the eye-tracking signal S1. The extractionof image features S12 from the test scene picture S11 is done in anextraction step P6 to be performed before the third processing step P3.In the third processing step P3, which includes estimating theperspective projection which compensates possible eye glass movements,the positions of the image features are used as inputs to the estimationalgorithm. Hence, to resolve a possible ambiguity of the solutions ofthe perspective projection estimation problem, the extracted imagefeatures take the role of the IR signal sources in case one or more ofthe latter are lost. The fourth processing step P4 is performed as inthe embodiment described above. It is noted that IR signals sources arein some respects superior to extracted image features in the role asreference points. The location of an image feature may be difficult toestablish with good accuracy; tracking the feature when the viewingangle of the test scene changes may prove delicate; and mostimportantly, the attention of the test subject may be distracted byimage features, which are, unlike IR light, visible. For these reasons,image features should be viewed merely as a supplement to the IR signalsources.

As an alternative to feature tracking, Kalman filtering can be used forreconstructing the position of the eye glasses in cases where the set ofreceived IR signals is incomplete. A Kalman filter for position andorientation estimation based on optical reference signals is disclosedin G. F. Welch, SCAAT: Incremental tracking with incomplete information,doctoral thesis, University of North Carolina, Chapel Hill (October1996), which is included herein by reference. The IR signal sourcetracking history is then used in association with assumptions on, e.g.,the maximal acceleration of persons wearing the eye glasses of thesystem. In a simpler approach, one may alternatively use the latestestimated perspective projection as an initial guess; apart fromaccelerating the estimation, this may also prevent the estimationalgorithm from converging to a false solution. As yet anotheralternative, if a history of observations is available around the pointin time having incomplete data (this implies that the processing isaccomplished non-causally), linear interpolation between two knownpoints may be used.

The presentation of gaze-point data detected by a system according tothe invention can be fashioned in a multitude of formats. As an example,FIG. 10a is a combined test scene picture with gaze points shown asdashed circles 1010, 1012, 1014, 1016, 1018. The circles 1010, 1012,1014, 1016, 1018 may correspond to momentary gaze points of differenttest subjects (‘bee swarm’ view). They may also correspond to gazepoints of one test subject at different points in time (‘gaze plot’view), provided the gaze-point detection covers a time interval ofnon-zero length. In this case, the circles 1010, 1012, 1014, 1016, 1018may be complemented by textual annotations to specify their respectivetime of viewing; they may also be connected by lines or arrows to showthe order of viewing, that is, the path of the viewer's gaze. Dots,stars, rectangles, crosses or other shapes may be used in the place ofcircles, as deemed appropriate in each application. The size of eachshape may be indicative of the accuracy of the detection, in that alarger shape may correspond to a greater numeric uncertainty, FIG. 10bis a test scene picture having superimposed shaded regions (‘heat map’view). Here, a deeper colour corresponds to a longer dwelling time ofthe gaze. Such dwelling time can have been accumulated over a testsession including only one test subject or may be the sum of dwellingtimes of a plurality of test subjects. According to an alternative datapresentation method, a plurality of areas of interest are predefined andthe dwelling time in each is accumulated during a test session includingone or more test subjects. The respective areas of interest are shadedin accordance with statistical measures of the dwelling times and aresuperimposed on a test scene picture (‘cluster view’). Statisticalmeasures include: mean total dwelling time per test subject, meanduration of each fixation, percentiles of the total dwelling time,percentiles of the test subject population having visited each area ofinterest, etc.

In a particular embodiment, test scene pictures are acquired at regulartime intervals (such as 10-100 pictures per second and suitably 20-40pictures per second) by the camera unit of the eye glasses. This way, avideo sequence of the test scene is obtained. The gaze point may then berepresented by superimposed graphical symbols, not on a stationary testscene picture but on respective frames of the video sequence of the testscene. (It is noted that even when only one test scene picture isacquired, the frames in such a video sequence could in principle bereconstructed on the basis of IR signal source tracking data by applyingperspective transformations to the one test scene picture; theassociated computational effort would be comparatively large.) Thisembodiment may be particularly useful in connection with simulators. Itis also advantageous in studies where the test scene is a video monitorpresenting a variable visual stimulus to the test subject. Indeed, acomprehensive evaluation of the results, in terms of the gaze point atdifferent points in time, should take into account what image the testsubject has been presented with at a given moment.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure and the appended claims. In the claims, theword ‘comprising’ does not exclude other elements or steps, and theindefinite article ‘a’ or ‘an’ does not exclude a plurality. The merefact that certain features are recited in mutually different dependentclaims does not indicate that a combination of these features cannot beused to advantage. Any reference signs in the claims should not beconstrued as limiting the scope.

The invention claimed is:
 1. A system for presenting gaze-point datacomprising: an eye tracking unit for detecting gaze point data of atleast one subject; a camera unit for viewing a test scene and acquiringa picture representing the test scene; a display; a processing unitconfigured to present the picture on said display, and display shapes onthe picture with the shapes representing momentary gaze points, detectedby said eye-tracking unit, of the at least one subject, with a size ofeach shape varying to indicate accuracy of the gaze-point data; saideye-tracking unit to acquire momentary gaze points over a time intervalof non-zero length; and said processing unit is further configured todisplay textual annotations specifying a respective time of viewing ofthe momentary gaze points, display lines or arrows connecting the shapesto specify an order of the momentary gaze points, accumulate the dwelltime of one or more subjects in each area of interest, and superimposeshaded areas of interest in accordance with statistical measures of thedwell times of the one or more subjects, with the statistical measurescomprising at least one of a mean total dwell time per test subject, amean duration of each fixation, percentiles of the total dwell time orpercentiles of a test population having visited each area of interest.2. The system of claim 1, wherein the shapes comprise at least one ofcircles, dots, stars, rectangles and crosses.
 3. A system for presentinggaze-point data comprising: an eye tracking unit for detecting gazepoint data of at least one subject; a camera unit for viewing a testscene and acquiring a picture representing the test scene; a display;and a processing unit configured to: present the picture on the display,and display shapes on the picture with the shapes representing momentarygaze points, detected by said eye-tracking unit, of the at least onesubject, with a size of each shape varying to indicate accuracy of thegaze-point data; said eye-tracking unit to acquire momentary gaze pointsover a time interval of non-zero length, with a color of the shapesindicating a dwell time of the at least one subject's gaze.
 4. Thesystem of claim 3, wherein the shapes represent a sum of dwell times ofa plurality of subjects.
 5. A system for presenting gaze-point datacomprising: an eye tracking unit for detecting gaze point data of atleast one subject; a camera unit for viewing a test scene and acquiringa picture representing the test scene; a display; a processing unitconfigured to present the picture on the display, and display shapes onthe picture with the shapes representing momentary gaze points, detectedby said eye-tracking unit, of the at least one subject, with a size ofeach shape varying to indicate accuracy of the gaze-point data; saideye-tracking unit to acquire momentary gaze points over a time intervalof non-zero length; and said processing unit is further configured topredefine a plurality of areas of interest in the test scene picture,display lines or arrows connecting the shapes to specify an order of themomentary gaze points, accumulate the dwell time of one or more subjectsin each area of interest, and superimpose shaded areas of interest inaccordance with statistical measures of the dwell times of the one ormore subjects, with the statistical measures comprising at least one ofa mean total dwell time per test subject, a mean duration of eachfixation, percentiles of the total dwell time or percentiles of a testpopulation having visited each area of interest.
 6. The system of claim5, wherein said processing unit is further configured to construct avideo sequence based on the test scene picture in combination with an IRsignal source tracking data provided by said eye tracking unit byapplying perspective transformations.
 7. The system of claim 5, whereinsaid processing unit is further configured to present a plurality oftest scene pictures acquired at regular intervals forming a videosequence, and display shapes on respective frames of the video sequence.